The term “sidelink” refers to a direct radio link for communication among devices in Third Generation Partnership Project (3GPP) radio access networks, as opposed to communication via the cellular infrastructure (uplink and downlink). The sidelink is also referred to as a Device-to-Device (or D2D) link. The term “V2x” denotes “Vehicle-to-Everything” communications, and it encompasses communications involving vehicles as message sources or destinations. V2x communications may involve respective vehicles as endpoints or may involve vehicles communicating with infrastructure nodes or other types of devices. The “PC5” interface is used to carry V2x communications between user equipments (UEs), which may be embedded in vehicles or other node, device, or system types engaged in Proximity Services (ProSe) D2D communications. The interested reader may refer to R3-150744—ProSe Resource Coordination Across eNBs, Ericsson, R3-132277—Mobility for D2D UEs, Ericsson, and 3GPP TS 36.423 Rel-13, for additional context.
Release 12 of the 3GPP specifications extend the LTE standard with D2D or sidelink communication features targeting both commercial and Public Safety applications. Some applications enabled by Rel-12 LTE are device discovery, where devices are able to sense the proximity of another device and associated application by broadcasting and detecting discovery messages that carry device and application identities. Another application consists of direct communication based on physical channels terminated directly between wireless devices. Here, the term “device” means a UE or essentially any apparatus configured for sidelink communications.
One of the potential extensions for the D2D work consists of support of V2x communication, which includes any combination of direct communication between vehicles, pedestrians and infrastructure. V2x communication may take advantage of a network (NW) infrastructure, when available, but at least basic V2x connectivity should be possible even in case of lack of coverage. Providing an LTE-based V2x interface may be economically advantageous because of the LTE economies of scale and it may enable tighter integration between communications with the NW infrastructure (V2I) and V2P and V2V communications, as compared to using a dedicated V2x technology. The nearby figure illustrates example V2x scenarios for an LTE-based wireless communication network.
V2x communications may carry both non-safety and safety information, where each of the applications and services may be associated with specific requirements sets, e.g., in terms of latency, reliability, capacity, etc. ETSI has defined two types of messages for road safety: Cooperative Awareness Message (CAM) and Decentralized Environmental Notification Message (DENM).
CAM messages are intended to enable vehicles, including emergency vehicles, to notify their presence and other relevant parameters in a broadcast fashion. Such messages target other vehicles, pedestrians, and infrastructure, and are handled by their applications. CAM message also serves as active assistance to safety driving for normal traffic. The availability of a CAM message is indicatively checked for every 100 ms, yielding a maximum detection latency requirement of <=100 ms for most messages. However, the latency requirement for Pre-crash sensing warning is 50 ms.
DENM messages are event-triggered, such as by braking, and the availability of a DENM message is also checked for every 100 ms, and the requirement of maximum latency is <=100 ms.
The package size of CAM and DENM message varies from 100 plus to 800 plus bytes and the typical size is around 300 bytes. The message is supposed to be detected by all vehicles in proximity.
The SAE (Society of the Automotive Engineers) also defined the Basic Safety Message (BSM) for DSRC with various messages sizes defined. According to the importance and urgency of the messages, the BSMs are further classified into different priorities.
UE mobility—i.e., cellular mobility within a cellular network—can be supported for UEs that are using PC5 resources. When such a UE is handed over, the PC5 resources with which it has been configured by its serving cell can be included by the source eNB in its “RRC context” (defined as RRC Context IE by 3GPP TS 36.423 and TS 36.331), signaled to the target eNB in the X2 HANDOVER REQUEST message. If the target cell is able to allocate the same set of radio resources to the UE, this prevents the UE from losing the PC5 connection. However, it is recognized herein that the UE still may experience unacceptably long interruptions in its sidelink communications during cellular handovers.
In typical D2D scenarios, PC5 synchronization in conjunction with cellular mobility may not be an issue due to the less stringent mobility requirements with respect to e.g. V2x, where high UE mobility is typical. As previously mentioned sidelink transmissions (also known as D2D or ProSe) in cellular spectrum that have been standardized in 3GPP since Rel-12 can be tailored for V2X-type services. In 3GPP Rel-12 two different operative modes have been specified in 3GPP. In one mode, a UE in RRC_CONNECTED mode requests D2D resources and the eNB grants them via PDCCH (DCIS) or via dedicated signaling. In another mode, a UE autonomously selects resources for transmission from a pool of available resources that the eNB provides in broadcast via System Information Block (SIB) signaling for transmissions on carriers other than the PCell or via dedicated signaling for transmission on the PCell. Therefore, unlike the first operation mode, the second operation mode can be performed also by UEs in RRC_IDLE.
A key issue recognized in this disclosure is that V2x traffic is characterized by messages with certain latency constraints. During a cellular handover of a UE there is typically some increase in latency due to the fact that a UE needs to read system information for the cellular network and perform a resynchronization and RRC reconfiguration to the target cell before being able to resume communication operations. Such operations may be regarded as performing a cellular handover process. The latency associated with the cellular handover process may violate some V2x traffic latency constraints.